Born in Victoria, Gozo on 11 November 1928, Mercieca was educated at the Gozo Seminary and studied at the Gregorian and Lateran universities, Rome, where he graduated in theology and canon law.
Ordained priest in March 1952, Mercieca was appointed chancellor of the Gozo Curia. He also taught theology and canon law at the Gozo Seminary, of which he was also rector for several years until his appointment as judge of the Sacred Roman Rota in 1969. He was also consultor to the Congregation of the Sacraments and of the Congregation for the Doctrine of Faith (Pro Doctrina Fide).
On 27 July 1974, Mercieca was appointed auxiliary to Archbishop Gonzi, being consecrated titular bishop of Gemelle in Numidia on 29 September 1974. He later became vicar general, a post he held until his nomination as metropolitan archbishop of Malta on 29 November 1976. His enthronement took place on 12 December 1976.
In June 1991 Mercieca was nominated member of the supreme tribunal of the Apostolic Signature.
In 1995 Mercieca was appointed member of the Xirka Gieh ir-Repubblika and was invested as conventual chaplain of the SMOM.
In 2003, Archbishop Mercieca sent his resignation to the Pope on his 75th birthday.